renault scenic 2004 - nightmare!!!?

totally fed up with this car, endless problems from rear window glass jsut slipping down - apparently happens to scenic cars- repair - the whole damn mechanism needs changing not just the runner bit!!
After weeks of cutting out whenever it felt like, it now just won't start for ages. RAC came, apparently cranshaft sensor & speed sensor fault - RACr approved garage at huge cost have put new one in & serviced car- they can't understand it - still won't start and keeps cutting out. Total madness - can anyone advise what i should do. surely if i sent the car to the garage to fix the car not starting i shouldn't have to pay for the new sensors if it hasn't fixed the problem- please advise